1. Field of the Invention
The present invention relates to passing routing information, such as used in the Open Shortest Path First (OSPF) protocol, and in particular to sending routing information on a multi-access network segment.
2. Description of the Related Art
Networks of general purpose computer systems and specialized devices connected by external communication links are well known and widely used in commerce. The networks often include one or more network devices that facilitate the passage of information between the computer systems and devices. A network node is a network device or computer or specialized device connected by the communication links. An end node is a node that is configured to originate or terminate communications over the network. An intermediate network node facilitates the passage of data between end nodes.
Communications between nodes are typically effected by exchanging discrete packets of data. Information is exchanged within data packets according to one or more of many well known, new or still developing protocols. In this context, a protocol consists of a set of rules defining how the nodes interact with each other based on information sent over the communication links.
A link-state protocol is an example of a routing protocol, which only exchanges control plane messages used for routing data packets sent in a different routed protocol (e.g., the Internet Protocol, IP). For example, the Open System Interconnection (OSI) protocol suite and the Open Shortest Path First (OSPF) routing protocol are link state protocols. A link-state protocol method floods routing information to all nodes in a controlled area of the internetwork in a link-state protocol control plane data packet. Link-state data indicates direct links between a network node that performs routing functions (a router) and a different router on the same network segment, which does not involve an intervening router. Link-state updates sent over a segment are acknowledged by other routers on that segment, for reliability. According to link-state protocol methods, each router builds a model of the entire area of the network in its routing tables based on the link-state data received from other routers.
When more than three routers share a network segment, the segment is a called a multi-access link. This is common in large networks and wireless networks. To save network resources (e.g., bandwidth on the multi-access segment and processing capacity on a network node), one of the routers on a multi-access segment is elected a designated router (DR) and a different router is designated a backup designated router (BDR). Instead of every router on the multi-access segment sending updates to every other router and receiving acknowledgments, each router sends update messages only to the DR and BDR. The DR resends the update message as a multicast to all nodes on the segment and waits for acknowledgements. If a node does not acknowledge the multicast within a configured time called the re-transmission time, then the DR sends a unicast to each unresponsive node. If the DR does not transmit a unicasts at the re-transmission time to an unresponsive node, then the BDR begins transmitting unicasts to the unresponsive nodes, which safeguards against a failure in the DR or its connection to the segment.
With current operations, including default values for the re-transmission time on the order of 5 seconds, the rescue of a failed DR by the BDR is not accomplished in times short enough for real-time data applications, such as voice and video conferencing.